Current revision as of 18:24, 13 June 2009

Ontologies and Rules

This chapter introduces “rules” as an alternative way of modeling knowledge
which complements the means of specifying knowledge we have already learned
about. In the broadest sense, a rule could be any statement which says that
a certain conclusion must be valid whenever a certain premise is satisfied,
i.e. any statement that could be read as a sentence of the form “if . . . then
. . . ” We will soon confine ourselves to concrete types of rules that will
be defined more accurately. Yet it is worth noting that the term “rule” as
such refers rather to a knowledge modeling paradigm than to a particular
formalism or language. And it is this paradigm that makes rules attractive
in many applications, since users sometimes find it more natural to formulate
knowledge in terms of rules than in terms of other kinds of ontological axioms.

But the difference between rules and ontologies is not merely pedagogical.
In the cases we consider, rules typically can help to express knowledge that
cannot be formulated in RDFS or OWL. At the same time, there are also
various features of OWL that rule languages do not provide, so a natural
question to ask is how the strengths of OWL and of rules can be combined.
It turns out that this is indeed possible, but that the added power often also
comes at the price of higher complexity and more difficult implementation.
The rule languages discussed in this chapter have therefore been chosen to be
the ones for which a combination with RDF and OWL is not just possible
in principle, but for which this combination is also practically supported by
software tools and, in some cases, by upcoming standards.